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954                           Appendix IV.

buildings, afterwards measuring the wind pressure on the various
surfaces. He found that the only cases in which the ordinary
methods of calculation for wind pressure on roofs (p. 471) agree
with practice are :

1.  A roof on columns, with free air space beneath.

2.  A roof lying on the ground, viz., without raised supports.
The other cases taken were :

3.   A roof supported on walls rising as far as the eaves.

4.   A roof on walls having parapets above the eaves.

In case 3, no pressure was experienced on the roof whatever,
while in case 4 there was actually a negative pressure, causing a
decided tendency to lift. These experiments, the results of which
are published in Vols. V. and VI. of the Australasian Association
for the Advancement of Science, are supported by our general
experience of roofs injured by wind.

CHAPTER IX.

Pp. 484 and 520. Mechanical Advantage and Velocity
Ratio.  Taking the symbols on p. 481, the theoretical form of the
principle of work, viz., neglecting friction, is

Px<* = WxD

But if Px is the practical effort required, including frictional effect,
the practical form of the principle of work becomes

Pj x d = WxD
and therefore the efficiency of the machine

Now Theoretical  mechanical advantage (T MA) = .  ; and

.           w

Practical mechanical advantage (P *M A) = =- ; and velocity ratio

d : .    .                                     >

= =r which is numerically equal to T M A.

W       W      P        d

       '                              1

ii                                                            = velocity ratio x ^